Disconnect and plug the vacuum advance and reset the timing to 35 degrees and report back on driveability. I had exactly the same problem with my big block, also with 3.55 gears, and disconnected the vacuum advance and the surging and detonation were gone. A good test is just to bring the revs up slowly in neutral, if it starts to miss, you have too much advance. Disconnect the vacuum advance and try it again.
